Native lignin is a naturally occurring amorphous complex cross-linked organic macromolecule that comprises an integral structural component of all plant biomass. The chemical structure of lignin is irregular in the sense that different structural units (e.g., phenylpropane units) are not linked to each other in any systematic order. It is known that native lignin comprises pluralities of two monolignol monomers that are methoxylated to various degrees (trans-coniferyl alcohol and trans-sinapyl alcohol) and a third non-methoxylated monolignol (trans-p-coumaryl alcohol). Various combinations of these monolignols comprise three building blocks of phenylpropanoid structures i.e. guaiacyl monolignol, syringyl monolignol and p-hydroxyphenyl monolignol, respectively, that are polymerized via specific linkages to form the native lignin macromolecule.
Extracting native lignin from lignocellulosic biomass during pulping generally results in lignin fragmentation into numerous mixtures of irregular components. Furthermore, the lignin fragments may react with any chemicals employed in the pulping process. Consequently, the generated lignin fractions can be referred to as lignin derivatives and/or technical lignins. As it is difficult to elucidate and characterize such complex mixture of molecules, lignin derivatives are usually described in terms of the lignocellulosic plant material used, and the methods by which they are generated and recovered from lignocellulosic plant material, i.e. hardwood lignins, softwood lignins, and annual fiber lignins.
Native lignins are partially depolymerized during chemical pulping processes into lignin fragments which are soluble in the pulping liquors and subsequently separated from the cellulosic pulps. Post-pulping liquors containing lignin and polysaccharide fragments, and other extractives, are commonly referred to as “black liquors” or “spent liquors”, depending on the chemical pulping process. Such liquors are generally considered a by-product, and it is common practice to combust them to recover some energy value in addition to recovering the cooking chemicals. However, it is also possible to precipitate and/or recover lignin derivatives from these liquors. Each type of chemical pulping process used to separate cellulosic pulps from other lignocellulosic components produces lignin derivatives that are very different in their physico-chemical, biochemical, and structural properties.
Given that lignin derivatives are available from renewable biomass sources there is an interest in using these derivatives in certain industrial processes. For example, lignin derivatives obtained via organosolv extraction, such as the Alcell® process (Alcell is a registered trademark of Lignol Innovations Ltd., Burnaby, BC, CA), have been used in rubber products, friction materials, adhesives, resins, plastics, asphalt, cement, casting resins, agricultural products, and oil-field products. However, large-scale commercial application of the extracted lignin derivatives, particularly those isolated in traditional pulping processes employed in the manufacture of pulp and paper, has been limited due to, for example, the inconsistency of their chemical and functional properties. This inconsistency may, for example, be due to changes in feedstock supplies and the particular extraction/generation/recovery conditions. These issues are further complicated by the complexity of the molecular structures of lignin derivatives produced by the various extraction methods and the difficulty in performing reliable routine analyses of the structural conformity and integrity of recovered lignin derivatives. For instance, lignin derivatives are known to have antioxidant properties (e.g. Catignani G. L., Carter M. E., Antioxidant Properties of Lignin, Journal of Food Science, Volume 47, Issue 5, 1982, p. 1745; Pan X. et al. J. Agric. Food Chem., Vol. 54, No. 16, 2006, pp. 5806-5813) but, to date, these properties have been highly variable making the industrial application of lignin derivatives as an antioxidant problematic.
Thermoplastics and thermosets are used extensively for a wide variety of purposes. Examples of thermoplastics include classes of polyesters, polycarbonates, polylactates, polyvinyls, polystyrenes, polyamides, polyacetates, polyacrylates, polypropylene, and the like. Polyolefins such as polyethylene and polypropylene represent a large market, amounting to more than 100 million metric tons annually worldwide. During manufacturing, processing and use the physical and chemical properties of certain thermoplastics can be adversely affected by various factors such as exposure to heat, UV radiation, light, oxygen, mechanical stress or the presence of impurities. Clearly it is advantageous to mitigate or avoid these problems. In addition, the increase in recycling of material has led to an increased need to address these issues.
Degradation caused by free radicals, exposure to UV radiation, heat, light, and environmental pollutants are frequent causes of the adverse effects. A stabilizer such as an antioxidant, anti-ozonant, or UV block is often included in thermoplastic resins for the purpose of aiding in the production process and extending the useful life of the product. Common examples of stabilizers and antioxidants include amine types, phenolic types, phenol alkanes, phosphites, and the like. These additives often have undesirable or even unacceptable environmental, health and safety, economic, and/or disposal issues associated with their use. Furthermore, certain of these stabilizers/antioxidants can reduce the biodegradability of the product.
It has been suggested that lignin may provide a suitable polymeric natural antioxidant which has a low level of toxicity, efficacy, and environmental profile. See, for example, A. Gregorova et al., Radical scavenging capacity of lignin and its effect on processing stabilization of virgin and recycled polypropylene, Journal of Applied Polymer Science 106-3 (2007) pp. 1626-1631; C. Pouteau et al. Antioxidant Properties of Lignin in Polypropylene, Polymer Degradation and Stability 81 (2003) 9-18. For a variety of reasons, despite the advantages, lignin has not been adopted for widespread use as an antioxidant. For instance, it is often problematic to provide lignins that perform consistently in terms of antioxidant activity. Also, the processing of the lignin may introduce substances that are incompatible for use with chemicals such as polyolefins. Additionally, the cost of producing and/or purifying the lignin may make it uneconomic for certain uses.